Figuring out whether Prime will be more of a savings or a spending enticement can be tough to gauge. Amazon has a complex system of shipping fees that vary by category and number of items per order.
Plus, the $35 free shipping minimum is often easy to hit. Two-day shipping rates range from $5.99 to $12.49 per shipment, making Prime pay off in as few as seven single-item orders or as many as 14.
"It won't be worth it to people that don't buy much," Levin said.
When it comes to streaming video, Amazon Prime works out to $16.88 cheaper a year than Netflix's $7.99-a-month unlimited streaming.
But the two companies have different libraries of titles, meaning value will vary by viewing habits. Watching Season Three of "Downton Abbey" is free for Prime members, while nonmembers pay either $1.99 per episode or $14.99 for the season—and more if they want to watch in HD. "Marvel's Avengers" is free for Prime, but nonmembers can only buy, not stream, the title, for $14.99.
With cases like that, a Prime membership could pay for itself in six movies or a TV series—except that many new releases aren't among the free titles for members.
"They're hoping as you get more familiar with using the Prime video offerings, you'll move into some of the new releases," Hottovy said.
—By CNBC's Kelli B. Grant. Follow her on Twitter
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